Mobile ad-hoc networks, or so-called MANETs, are different from organized or structured networks such as available for the commercial wireless segment.
The mobile ad-hoc nature of the network means that there is no in place infrastructure. Those who want to communicate have radios that could work on a point to point basis if they are in range. The problem is extending the range of the mobile radios by having radios that forward data from one node to the other node in a multi-hop fashion such that the data is repeated from one radio to the other intermediate radios until it reaches the final destination.
For instance if one wishes to send pictures or video one can do so on an ad-hoc basis without prior coordination by using a string of intermediate radios to facilitate end-to-end communication. It will be appreciated that the radios in the ad-hoc network establish a point-to-point communication and do this over network links between nodes. In so doing there is extra logic that enables packets of data to be forwarded depending on where they are to go. Thus, in an ad-hoc network typically this is not a pure broadcast situation. Rather there is a directed point to point movement of packets through a chain of point-to-point moves. Note that when data is transmitted from one node to another one does this in a single hop over a link which is why mobile ad-hoc networks have a link layer so that the communication occurs with one hop between two nodes within range over the corresponding link.
If it is desired to send information to a node that is not within range one invokes the cooperation of other nodes that would store and forward the data so that it gets from one point to an end point. All of the node-to-node linking is controlled with logic that organizes the transmission of the data between a number of radios, each one being referred to as a node within the networking network that cooperate to pass data along beyond the reach of any particular radio.
This multi-hop end-to-end transmission in an ad-hoc network suffers from a number of problems. First and foremost is that the overhead involved in providing so-called link state lists to control the node-node transmissions. This overhead prevents the number of nodes from exceeding a very small number, typically not greater than 30 nodes. Prior ad-hoc networks have operated adequately for tens of nodes. However, a hundred or more nodes stretches the bandwidth of the overall network. The challenge therefore is to be able to scale an ad-hoc network to a thousand or more nodes.
One of the major requirements for ad-hoc networks is in the battlefield where for instance one requires a battalion size communication system in which perhaps there is as many of 1,000 troops each having their own individual radio. In a battalion situation there will be a significant number of participants that want to be able to communicate between themselves, rather than just communicating with small nearby groups of such radios or nodes. As will be seen, the primary impediment to scaling ad-hoc networks up to 1,000 nodes is the massive amount of overhead involved in transmitting link state messages to update all of the radios within the network.
As will be appreciated, the challenge is to be able to cover large number of nodes in wide areas without infrastructure available from commercial cellular networks. In a cellular network where the infrastructure is in place in order to cover more and more nodes in more and more places one simply increases the power or provides more cells and cables in between the towers. The landline or cables between the towers can carry an infinite amount of overhead, at least as compared with the limited bandwidth available for a tactical mobile radios.
Note, in a tactical situation it is time wise not feasible to install all the infrastructure that has to be up and running up front for a large population of radios or users.
Rather than a fixed infrastructure system, when trying to implement an ad-hoc system, one typically prefers a routing system for hop by hop routing of data between network nodes. It is a characteristic of a mobile ad-hoc network that when the network becomes larger and larger on average one needs more hops per unit of data. This is because one wants to communicate not with just immediate neighbors, but with nodes that are on average farther and farther away. Thus one needs to add more and more hops to communicate end-to-end over large distances. In order to implement such a system one needs more and more radios for the same data. However adding nodes to the network becomes more and more expensive in terms of bandwidth such that there are higher and higher levels of data that need to be transmitted for the same unit of data to be delivered to the end destination.
The result is the average throughput through the network decreases as one increases the number of nodes. Thus when one increases the nodes one moves less data because per unit of data one is using more overhead. For instance if it takes a tenth of the time to transmit data on a first hop and a tenth on the second hop and so on, this means that all nodes and links will have less bandwidth for something else. By moving one packet through more and more nodes there is more and more waste of resources. Moreover, the total overhead resources can be used up in an effort to just move one piece of data through a number of nodes. If one overloads the system with too much overhead the result is lost data. This means that as one increases the nodes in the network one has to be content with a less amount of data per unit of time on average through the network. Alternatively, if one wants to push more data through the network one will simply lose more data. Thus there is a tradeoff between the amount of data that needs to be pushed versus the bandwidth associated with the number of nodes necessary to push the data.
For instance in a tactical situation in which one wishes to transmit video data at a medium resolution the data could be streaming video having a bandwidth of 1 megabit per second. On the other hand, the raw bandwidth of a radio transmitting the video could be perhaps 10 megabits per second. Each radio would then utilize 10% of the available raw network bandwidth. Thus for instance one could conceive that on average the video transmission represents a tenth of the capacity of a radio. However if one seeks to transmit the video between 10 nodes, one needs to take 10% of the bandwidth associated with each radio to go to the other side. At 10 hops this means going through 10 nodes to receive and transmit. Thus going through all of the 10 nodes utilizes 100% of the network bandwidth just for overhead.
In actuality each radio in this case would consume 20% of the bandwidth. This is because as one goes hop to hop each radio within range will hear the transmission which occupies 10% of the bandwidth. However each node both transmits and receives so that from a transmission and reception point of view each radio for each hop utilizes 10% to receive and 10% to transmit for a total of 20%. In the above example this means that the total capacity of the network will be reduced by 200%, twice the bandwidth of one radio.
Of course in operation the totality of the bandwidth in the whole network is larger but the point is that each radio uses up a fractional portion of the bandwidth for each radio. Therefore each of the radios on the path will have less capacity to transmit and receive data to and from other radios. It is noted that the more bandwidth that is utilized by the network, the less bandwidth per node on average will be available for instance for future extra video or some other type of communication. Thus the available throughput per node on average will decrease as the size of the network increases.
The commercial solution to such a conundrum is to divide a network into smaller pieces and provide one piece of infrastructure or cellular tower for each small cell. Note that in a cellular system overhead does not contribute to the bandwidth because it is carried over almost unlimited bandwidth optical cable between cell towers.
In short, in a simple ad-hoc network the adding of nodes or radios quickly overwhelms the network with overhead. This is a severe problem which has not been solved up to the present.
Another persistent problem with ad-hoc networks is the need for the ad-hoc network to organize. Ad-hoc networks are not organized in advance. They need to organize by establishing routing or paths or directions for the data to go, any time one wants to send data from one node to another. Routing in general is done through a number of known protocols which specifies how to send the data packets, the most popular of which is the Optimized Link State Routing (OLSR) protocol.